1. Field of the Invention
This invention relates to a substrate for a liquid crystal display device that constitutes a display device such as information equipment and the like, and to a liquid crystal display device equipped with the same.
2. Description of the Related Art
There has heretofore been known a liquid crystal display device of an MVA mode (multi-domain vertical alignment mode) by vertically aligning liquid crystals having a negative dielectric anisotropy and having a bank structure (linear protuberance) and a slit of electrode on a substrate as an alignment regulating structure. Being provided with an alignment regulating structure, it is allowed to control the alignment of liquid crystals in a plurality of directions of when a voltage is applied even without rubbing the alignment film. The liquid crystal display device of the MVA mode is superior in viewing angle characteristics to the liquid crystal display devices of the conventional TN (twisted nematic) mode.
Another method of regulating the alignment of the liquid crystal molecules may be a polymer fixing system. According to the polymer fixing system, a liquid crystal composition obtained by adding a trace amount of a monomer that polymerizes upon the irradiation with ultraviolet rays (UV) to the liquid crystals having a negative dielectric anisotropy, is sealed between two pieces of substrates having vertically alignment films formed on the surfaces thereof facing each other. Thereafter, a predetermined voltage is applied across the substrates to tilt the liquid crystal molecules which are, then, irradiated with UV to polymerize the monomer.
FIG. 18 is a view schematically illustrating a state of a liquid crystal composition in which the monomer is polymerized. As shown in FIG. 18, electrodes 114 and 115 are formed on the opposing surfaces of two pieces of glass substrates 112 and 113. Vertically alignment films 116 and 117 are formed on the electrodes 114 and 115.
A liquid crystal layer 118 between the two substrates is constituted by liquid crystal molecules 102 aligned nearly vertically to the surfaces of the substrates and polymer structures 106 formed by the polymerization of monomers 104 that are tilted at a given pre-tilted angle. The polymer structures 106 are formed as the liquid crystal layer 118 is irradiated with UV in a state where a predetermined voltage is applied across the electrodes 114 and 115 from a power source 140 through a switch 142. Since the polymer structures 106 are fixed at a given pre-tilted angle with respect to the surfaces of the substrates, the surrounding liquid crystal molecules 102 are caused to be tilted in a predetermined direction. Therefore, the liquid crystal display device of the MVA mode based on the polymer fixing system does not require an alignment regulating structure that was so far used for forming a multiplicity of domains and, hence, offers an advantage of simplifying the production process.
Further, the polymerized liquid crystal molecules have a lower threshold voltage than that of other liquid crystal molecules (host liquid crystals), and respond quicker than the other liquid crystal molecules when driven on a low gray level; i.e., the response time of the liquid crystal molecules is improved on a low gray level.
According to the polymer fixing system, the liquid crystal layer 118 must be irradiated with UV while being applied with a predetermined voltage to impart a given pre-tilted angle. In order to apply a voltage to the liquid crystal layer 118, a predetermined voltage (gray-level signal) must be applied to the data bus line and another predetermined voltage must be applied to the gate bus line to turn on the thin-film transistors (TFTs) of the pixels.
The voltages are applied to the bus lines usually by a probing method or a method which utilizes a peripheral connection line (common wiring). The former method uses a contact prober or the like to probe terminals of bus lines which are electrically independent from each other to apply a predetermined voltage thereto. The latter method applies a voltage by using the peripheral connection line that has been electrically connected to a plurality of bus lines.
FIG. 19 illustrates a state where a voltage is applied to the bus lines 120 by probing. As shown in FIG. 19, a contact prober (not shown) has a plurality of probing pins 122 that are brought into contact with the terminals 124 of the bus lines 120 to apply a predetermined voltage to the bus lines 120.
As the wiring patterns are highly finely formed in recent years, however, the gap P among the bus lines 120 is becoming narrow making it difficult to conduct the probing. Further, the number of the required probing pins 122 increases with an increase in the number of the bus lines 120, and it is becoming more difficult to reliably bring all of the probing pins 122 into contact with the terminals 124.
In FIG. 19, the third probing pin 122′ from the left is not in contact with the terminal 124′. Without reliably effecting the probing, the predetermined voltage is not applied to the pixels on the bus line 120′ connected to the terminal 124′, and the liquid crystal molecules 102 are not controlled for their pre-tilted angles to a sufficient degree by the polymer fixing system. Therefore, the defective display (linear defect) occurs on the bus line 120′ deteriorating the quality of display.
FIG. 20 illustrates a constitution of a peripheral connection line used for applying a voltage. As shown in FIG. 20, the terminals 124 of the bus lines 120 are electrically connected together through a peripheral connection line 126. A pad 128 for applying voltage is electrically connected to the peripheral connection line 126. Upon bringing a probing pin (not shown in FIG. 20) into contact with the pad 128 for applying voltage, a predetermined voltage is applied to a plurality of bus lines 120.
However, the bus lines 120 which are electrically connected together must be electrically separated from each other in a step after the steps of producing a panel. Therefore, an extra step is necessary for either effecting the secondary scribing to cut the glass substrate along a dot-dash chain line α or cutting, by a laser, the wiring between the peripheral connection line 126 and the terminals 124 along the dot-dash chain line α, arousing a problem of an increase in the cost of production.